In wire drawing machines of the foregoing type, a coil of wire is wrapped around a rotatable drum. When the drum is rotated, wire is drawn through a die and wound onto one end of the drum and, at the same time, wire is unwrapped from the other end of the drum and is delivered to the production machine. The speed of rotation of the drum must be matched to the demand of the production machine and, for this purpose, the drum is rotated by a variable-speed drive mechanism which may be adjusted manually to cause the drum to rotate at a selected speed. To adjust the speed of the drum in response to changes in the rate of wire consumption by the production machine, a pivoted compensator arm is biased into engagement to a loop of wire formed after the wire leaves the drum. When the speed of the drum is too great, the size of the loop increases and the arm swings in one direction to either shut off or slow down the drive to the drum. If there is insufficient supply of wire, the loop decreases in size and causes the arm to swing in the opposite direction so as to either restart the drive or increase its speed.
In those instances where the variable-speed drive mechanism is of the mechanical type, the adjustment of the speed of the drum can only be made over a portion of the speed range of the drum. The operator of the machine typically begins operation by adjusting a hand crank so as to cause the mechanical variable-speed drive mechanism to rotate the drum at a speed at which the drum supplies just slightly more wire than is being consumed by the using station. As a result, the loop tends to increase in size and permits biasing springs to pivot the compensator arm clockwise. Such clockwise pivoting of the compensator arm is mechanically linked to a commercially available variable-speed mechanical drive mechanism. In response to the pivoting of the compensator arm, the mechanical linkage changes the effective diameter of one of the pulleys in the drive mechanism so as to change the drive ratio between an electric drive motor and the drum.
A variable-speed drive mechanism of the foregoing type is typically mounted on the frame of the in-line drawing machine. The adjustable-diameter pulley is directly driven by the electric drive motor and is connected by an endless belt to a similar adjustable pulley mounted on a shaft which serves to drive a speed reducer. The output shaft of the speed reducer is connected to the drive shaft of the drum by a second endless belt and by additional pulleys.
A significant problem with the conventional variable-speed drive mechanism is that tension or a "pull-back" force is developed on the wire as the linkage joining the compensator arm of the drive mechanism attempts to change the effective diameter of the pulleys. Specifically, as the diameter of the top pulley in the variable-speed mechanical drive is made larger, an axially slidable disc of the bottom pulley is pressed into a biasing spring. The force of the spring is fed back to the compensator arm such that a "pull-back" force is exerted on the wire, thereby creating tension along the length of the wire as it is supplied to the using station. Such tension is highly undesirable since it may result in the wire pulling away from the using station and causing the production machine to be severely damaged. For example, the dies used in cold headers can be ruined if a stamping operation occurs without wire correctly positioned between the halves of the die. Too much tension in the wire may cause the end of the wire to pull away from its proper position in the die--hence, the name "pull-back" force.
In order to avoid tension in the wire and the associated risk of damaging the production machine fed by an in-line wire drawing machine, the previously mentioned '403 patent to Alcock employs a mechanism for limiting the speed adjustment of the drum to a range centered about the nominal consumption rate of the production machine. By providing for a limited speed adjustment, the demand by the linkage for changes in the effective diameter of the pulley is limited, thereby also limiting the magnitude of the "pull-back" force exerted on the wire.
Limited speed adjustment may avoid placing too much tension on the wire, but it requires constant monitoring for the possible need of manual adjustment of the initial set up of the wire drawing machine, and such monitoring and manual adjustment is undesirable. In this regard, these machines are often manned by a relatively unskilled work force, and the availability of only a limited speed adjustment requires that these machines be carefully watched in case a speed adjustment outside of the range of an automatic adjustment becomes necessary. In such a case, the nominal speed of the machine must be adjusted by hand in order to ensure the required connection remains within a predetermined range of the nominal value.